Brown Booby

About

The Brown Booby is a large seabird belonging to the gannet and booby family, Sulidae. It is characterized by its striking dark brown upperparts and contrasting white underparts, a pattern that gives it its common name. These birds are highly adapted for plunge-diving to catch fish and squid, often seen soaring over tropical and subtropical oceans.

Size & lifespan

Length

64-76 cm (25-30 in)

Wingspan

130-150 cm (51-59 in)

Weight

700-1500 g (1.5-3.3 lbs)

Lifespan

Up to 20 years in the wild

Diet

Carnivore (primarily fish and squid)

Physical characteristics

Plumage

Adults have dark brown plumage on the head, neck, upperparts, and upper breast, sharply contrasting with pure white lower breast, belly, and underwing coverts. The flight feathers (primaries and secondaries) are dark brown. Juveniles are entirely dusky brown, gradually acquiring the adult pattern.

Beak shape

Conical, pointed, and relatively stout. The bill color varies with age and sex: adult males typically have a bluish-gray bill base with a yellowish tip, while females have a more yellowish or greenish-yellow bill. Juveniles have a dusky bill.

Eye color

Yellowish-white to pale yellow in adults, darker in juveniles.

Leg color

Dull yellow to greenish-yellow, with webbed feet.

Distinguishing features

The sharp demarcation between the dark brown upperparts and white underparts is the most distinctive feature. The dark brown extends onto the upper breast, forming a 'hood' or 'vest' appearance. The pale bill and feet are also key.

Juvenile appearance

Juveniles are uniformly dusky brown over their entire body, including the underparts, which are mottled with white. They lack the sharp contrast of adults. Their bill and feet are also darker, becoming paler with age.

Seasonal changes

No significant seasonal plumage changes. Plumage remains consistent year-round once adult coloration is achieved.

Sexual dimorphism

Subtle. Females are generally slightly larger than males. The bill color can differ, with males often having a bluer tint at the base of the bill and females a more yellowish or greenish-yellow bill. The bare skin around the face (lores) is typically yellow in males and greenish-yellow in females.

How to identify

1. 1

   Plumage Pattern

   Look for the sharp contrast between the dark brown head, neck, and upperparts, and the pure white lower breast and belly. The dark brown extends like a 'vest' onto the upper breast.

2. 2

   Bill and Feet Color

   Note the pale bill (yellowish to greenish-yellow, sometimes with a bluish base in males) and dull yellow to greenish-yellow webbed feet. This distinguishes it from Red-footed Booby (red feet) and Masked Booby (black flight feathers, black mask).

3. 3

   Size and Shape

   A medium-sized booby with a streamlined body, long pointed wings, and a wedge-shaped tail. Its flight is powerful and direct.

4. 4

   Juvenile Identification

   Young birds are entirely dusky brown, lacking the adult's white underparts. They will gradually acquire the adult pattern over several molts.

5. 5

   Behavior

   Observe its plunge-diving feeding technique. It dives headfirst into the water from a considerable height to catch fish.

Habitat & range

Biomes

Marine, coastal, pelagic

Geographic range

Widespread in tropical and subtropical oceans globally, including the Atlantic, Pacific, and Indian Oceans. Found in coastal waters, offshore islands, and open ocean.

Elevation range

Sea level to several hundred meters above sea level (when nesting on cliffs or islands). Primarily marine.

Preferred environments

Warm oceanic waters, especially near continental shelves, oceanic islands, and coral reefs. They prefer areas with abundant schooling fish and often roost on buoys, ships, or small islands.

Behavior

Activity time

Diurnal, actively foraging during daylight hours.

Feeding habits

Primarily piscivorous, feeding on small to medium-sized schooling fish (e.g., flying fish, sardines, anchovies) and squid. They are spectacular plunge-divers, often diving from heights of 10-30 meters (30-100 feet) with wings partially folded, entering the water with a loud splash. They typically catch prey close to the surface.

Social behavior

Colonial breeders, forming large nesting colonies on remote islands and cliffs. Outside the breeding season, they can be seen singly or in small groups, often associating with other seabirds or fishing vessels. They are generally not highly social at sea.

Flight pattern

Strong, direct flight with powerful wing beats, interspersed with long glides. They often fly low over the water, utilizing updrafts. They are agile in the air, especially when plunge-diving, but can appear somewhat clumsy on land.

Migration pattern

Generally considered resident or dispersive rather than truly migratory. Adults tend to remain in tropical waters year-round, but juveniles and non-breeding birds may disperse widely from their natal colonies, sometimes traveling thousands of kilometers.

Territoriality

Highly territorial around their nest sites within breeding colonies, defending a small area around the nest. At sea, they are not territorial.

Breeding & reproduction

Breeding season

Varies geographically, often year-round in some tropical regions, but typically peaks during specific seasons (e.g., spring/summer) in others, often coinciding with food availability.

Nesting habits

Nests are typically shallow scrapes on the ground, often lined with pebbles, shells, or vegetation, located on remote oceanic islands, rocky islets, or cliffs. They nest in dense colonies.

Clutch size

1-3 eggs, typically 2.

Egg description

Usually 1-3 eggs (most commonly 2), chalky white or pale bluish-white, often stained brown from the nest material. Eggs are elliptical to oval, measuring approximately 59-68 mm x 40-46 mm.

Incubation period

40-45 days, shared by both parents.

Fledging period

90-100 days. Young are altricial (helpless at birth) and remain in the nest for an extended period, fed by both parents.

Parental care

Both parents share incubation duties and care for the chicks. Only one chick typically survives if two hatch, due to siblicide (the stronger chick often outcompetes or kills the weaker one).

Where to find

Field tips

1. Scan the Horizon. When at sea in tropical waters, continuously scan the horizon for birds flying low over the water or plunge-diving. Their distinctive plumage makes them stand out.
2. Look for Fishing Activity. Brown Boobies often congregate where fish are schooling, sometimes alongside other seabirds or dolphins. Look for signs of feeding activity.
3. Visit Breeding Colonies. If visiting a tropical island known for seabird colonies, look for them nesting on cliffs or open ground. Maintain a respectful distance.
4. Observe Plumage Details. Pay close attention to the extent of the dark brown plumage on the breast and the color of the bill and feet to differentiate from similar booby species.

Similar species

Quick facts

Diet:

Fish and squid

Habitat:

Tropical and subtropical oceans, coastal areas, islands

Nesting:

Colonial ground nester on islands

Conservation Status:

Least Concern

Distinguishing Mark:

Dark brown upperparts and upper breast, sharply contrasting with white lower breast and belly

Fun facts

• Brown Boobies are excellent plunge-divers, hitting the water at high speeds to catch their prey. Their nostrils are permanently closed, and they breathe through their mouths, an adaptation for diving.
• They are one of the most widespread booby species, found in all three major tropical oceans.
• Like other boobies, they have webbed feet that are used for swimming and also for incubating eggs, as they lack a brood patch.
• In many colonies, siblicide is common, where the first-hatched chick often kills its younger sibling, ensuring that at least one chick receives sufficient resources.

Scientific classification

Kingdom

Animalia

Phylum

Chordata

Class

Aves

Order

Suliformes

Family

Sulidae

Genus

Sula

Species

Sula leucogaster

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